The invention relates to metrology which in turn involves the technology of the use of reflected light in the investigation of the bulk and surfaces of solids and solid materials; and in particular to the extension of the sensitivity and detectability achievable in that technology.
Metrology involving the use of reflected light in the identification or characterization of the surface and bulk aspects of solid materials has been receiving considerable attention over a long period in the art. In essence, at the present state of the art, there are two general types of characterization technologies. In a first type, the light is incident to and reflected from the sample under study at a direction that is approximately the vertical or the normal with respect to the surface of the sample. The polarization induced differences in the reflected light that are produced by the sample are the measured quantity. This type of characterization has acquired the name in the art of Reflection Difference Anisotropy and is sometimes identified by the acronym RDA. In a second type, the light is incident to and reflected from the sample under study at relatively large angles with respect to the sample, as compared with the first type. Changes in polarization of a reference polarized light resulting from variations of the angles, and changes in light both in and out of the plane of the device are the measured quantities. This second type includes most ellipsometry techniques.
In a less constrained form of ellipsometry, called herein BiDirectional Ellipsometry, the incident and reflected angles need not be equal. One determines the amplitude and polarization of the reflected beam as a function of the polarization, wavelength and angle of incidence of the incident beam, as well as of the deviation of the measured direction from the specular (defined as equal to the angle of incidence) beam. This deviation being defined in the two directions of xe2x80x9cwithinxe2x80x9d and xe2x80x9cperpendicular toxe2x80x9d, the plane of incidence. While the principles are useful in many types of characterization measurements, each situation in which the principles are applied will involve a number of considerations that are unique to that situation. In order to establish a perspective in the application of the technology; the semiconductor industry is used as an illustration for the reasons that the technology is quite amenable to such considerations as the dimensions being small, that the characterization of the material under study usually be non-destructive and that there be an ability to make monitoring determinations in real time.
In the semiconductor industry the principles of ellipsometry have been extensively applied to the types of material characterizations needed to meet the ever decreasing dimensions encountered in devices and in their fabrication.
In the 1980 timeframe, P. S. Hauge in an article titled xe2x80x9cRecent Developments in Instrumentation in Ellipsometryxe2x80x9d in Surface Science, Vol. 96 (1980), pages 108-140 provided a survey of the instrumentation available in the art at that time.
In the 1988 timeframe, D. E. Aspnes, in an article titled xe2x80x9cAnalysis of Semiconductor Materials and Structures by Spectroellipsometryxe2x80x9d in SPIE Vol. 946III pages 85-97; provided an anthology of the state of the art at that time for various types of material characterizations and data analytical techniques.
Also in 1988, R. L. Johnson et al, in an article titled xe2x80x9cSpectroscopic Ellipsometry with Synchrotron Radiationxe2x80x9d in Review of Scientific Instruments 60, 7, July 1988 Pages 2209-2212 described the use of high photon energy syncrotron radiation in ellipsometry on InP, Y Ba2Cu3O7 and CaE2. By the approximately 1994 timeframe, U.S. Pat. No. 5,526,117 indicated that the art had progressed to dimensions in the vicinity of the 100 nanometer (nm) range.
In 1997, Germer et al. in an article titled xe2x80x9cPolarization of out-of-plane scattering of microrough siliconxe2x80x9d, in Optics Letters, Vol. 22 No. 17, Sept. 1997, pages 1284-1286, indicates the advantages of polarization information that is out of the plane of the device, i.e., BiDirectional. ellipsometry.
There are metrology companies such as Sentech in Germany, and Woollam in the U.S., that have strong programs and ellipsometric metrology products in the marketplace.
The present technology has been yielding satisfactory results where the dimensions under study are above tens of nanometers (nm) but it is becoming increasingly difficult to get the accuracy needed. Current expectations are toward gate widths of the order of 0.25 micrometers in 1998, progressively becoming narrower to about 0.1 micrometers by 2007. Dielectric thicknesses must shrink to meet the line widths for a gain in performance to be realized. The dielectric thickness must therefore shrink from about 4.0 nm in 1998 to about 1.5 nm in 2007. Further such dimensions will probably be made up of several thinner layers. Metrology is a necessary corrolary to achieving these technologies. It is necessary to be able to measure a dimension in order to be able to properly control it. The thicknesses and tolerances predicted cannot be properly characterized with current technologies. The limitations of the equipment presently in use in the art has resulted in photon energy ranges of less than 6 eV, beyond which the light is not transmitted to the detector and the ability to distinguish is limited. Such a situation is present in the semiconductor industry wit the types of dielectrics being used. Progress at this point in time however is primarily being directed to using extended range reflectometry applied to current techniques, and the goals are primarily to characterize optical properties at lithography wavelengths such as 193 nm.
Improvements is accuracy and sensitivity in the optical characterization of solids and solid materials are achieved through the use of the interdependent features of: extending the photon energy range over which the metrology is performed to include; the range up through about 10 eV, in which, the higher photon energy of the light improves signal distinguishing ability; providing a controlled ambient in the entire light path between the light source and a detector that prevents absorption and signal definiteness masking so as to sharpen the identifiability of the change parameters imparted into the reflected light. There are provided combinations of specific devices and materials that define the features in the characterization.